Several Companies make a Forend for the 870, and the Mossberg 500/590with a built in White Light Flash Light. They are not cheap.
I have also seen clamp on barrel light mounts.
They also make a Tritium replacement for the Bead front sight.
I have a Pre Wilson Combat ScatterGun Technoligy TR870. It has there Trac Loc sights. Tritium Front Sight, and Ghost Ring back sight.
The built in flash light forend with a pressure switch is probably your best place to start.